Transmission of data using optical fiber is a well known and advantageous method of communication. Optical fibers are generally made of silica glass containing a central core in which the light travels.
Each fiber requires proper termination so that joints can be made, for example, where 2 fibers are spliced together or where optical transmitters, detectors or connectors require attachment to the fiber. Proper termination requires that an end surface of the core of the fiber is left substantially mirror smooth, at a predetermined angle to the axial centre of the fiber.
Construction of the optical splice requires that the core of the optical fibers should be brought closely together so that light travelling in one fiber is required to traverse as small a gap as possible to enter in to the mating fiber, the gap being small so that there is little opportunity for the light to spread out in the intervening gap and consequently as much light as possible is injected in to the mating fiber—in other words to minimize the Insertion Loss of the optical splice.
The use of perpendicularly terminated cleaved end faces ensures the closest possible approach between the two fiber ends so that light travelling in one fiber or fiber array can traverse in to the second fiber or fiber array with substantially no loss arising from the light spreading out in the intervening gap. The perpendicularly terminated end can be prepared by polishing, mechanical or laser cleaving or otherwise. However, a substantial fraction of up to 4% of the incident light is back-reflected from a mirror-smooth end face and if the end face is perpendicular, this reflected light is deleteriously transmitted back down the fiber, creating optical noise and feedback. The gap between the two fiber ends may be filled with index matching gel but this is only effective in suppressing reflections over a limited temperature range. It has therefore become industry standard practice to angle the fiber ends to ensure that any reflection is not transmitted down the optical fiber; an 8° angling away from the perpendicular reduces the transmitted back-reflection from −14 dB to −60 dB. This elimination of optical noise from the transmission system is of great advantage when pigtailing optical fibers to photonic components or when mechanically splicing two lengths of fiber together.
Cleaving tools to create a cleaved fiber end are well known in the art, such as disclosed in EP 295,374 B and manufactured by Fujikura KK of Japan, and work by a combination of clamping, scratching and bending the optical fiber and produce a substantially perpendicular cleave. A side view of a perpendicular cleaved end is shown in FIG. 4a. To achieve an angle cleaved end, the fiber may be twisted prior to cleaving, as in tools manufactured by PK Technologies, USA or by Ilsin Technologies, South Korea. The resultant cleaved end face arising from such twist-to-angled-cleaves techniques have a “nose” of glass projecting beyond the core of the fiber, as shown in FIG. 4f, and this hinders close approach of the cores of two fibers when they are joined together in a mechanical splice as it defines a minimum distance therebetween; FIG. 6e shows two such fibers joined together in a splice in an orientation in which the separation (2×A t) of the two cores is a maximum.
Angled cleaves may also be created by bending a tensioned fiber or fibers to create internal shear stresses. This technique has the advantage that the cleaved end face is smooth without the roughness associated from ripping. The end angle achieved is set by the magnitude of the deflection of the fiber between two known points and so the end angle achieved is very repeatable. Importantly, such bend-to-angle-cleave techniques have a very much reduced “nose” of glass, N, projecting beyond the core of the fiber, in which the core of the fiber is angled but the portion of the cleaved end face close to the scratch created by the blade substantially perpendicular to the optic axis of the fiber (see FIG. 2a and FIG. 4b). Such an angled cleaved end with a reduced “nose” can more closely approach the waveguides in a “pig-tailed” opto-electronic component such as a laser diode or a waveguide because of the reduced glass projection. There are a multitude of opto-electronic devices such as laser diodes, optical detectors, planar light-guides or other, with which an optical fiber interfaces so that the light can enter or exit the opto-electronic component. The core of the optical fiber should approach closely, or abut, the waveguides within the component. Increased separation of the core of the fiber and the component gives a significant insertion loss as the light spreads out as it travels unguided in the intervening air gap. In many cases, there is also an advantage to angling the cleaved fiber end to prevent optical reflections travelling in the opto-electronic component or optical fiber. Therefore, it is advantageous if the fiber end is angle cleaved with a reduced glass projection.
U.S. Pat. No. 6,578,747, in the name of Murgatroyd, shows, at FIG. 4a, the result of angle cleaving an optical fiber or fibers by shearing tensioned fiber or fibers between two sharp corners. The core of the fiber is angled but the region of the fiber close to the cleave-initiation scratch is substantially perpendicular to the optic axis of the fiber, giving a reduced glass projection beyond the core of the fiber. If such angle ends are used in optical splices, the core of the fiber can closely approach a second optical fiber because of the reduced glass projection.
In contrast, angling the ends of one or both fiber or fibers or array of fibers will prevent close approach between the cores of the optical fibers constituting the optical splice owing to the mechanical shape of the “nose” of the cleaved fiber end, as shown in FIG. 6e. This separation of cores can lead to insertion losses of 3 dB or more, even when the intervening gap is filled with index matching gel, 90. Of course, close approach of the cores of the fibers can be achieved by ensuring that the two angled fiber ends are oriented so that their angled ends are mated, so substantially reducing the core separation and so minimising the insertion loss, but this requires control of the orientation of both fibers. Such requirement of control of the fiber orientation is expensive and difficult to achieve.
Recent developments by Gurreri (U.S. Pat. No. 8,104,974) have shown that this close control of orientation of the angled fiber ends can be eliminated if the angled fiber ends are angled but not planar. The core of the fiber is arranged so that it is angled at 1°-30° away from the perpendicular, and preferably 4°-12° and most preferably close to 8° from the perpendicular and this suppresses the back-reflection to −60 dB or less. The “nose” normally expected from a planar angled end is missing and so the cores can closely approach each other to reduce insertion loss and so the cores of the fibers can approach each other closely, whatever the rotational orientation of either fiber.
It can be seen that close approach of the angle cleaved fiber end to the optical fiber or opto-electronic component is widely beneficial and this can be achieved by angle cleaving by shearing the fiber.
Optical fiber splicing is often carried out in the field and so one or both of the optical fibers must be terminated in the field. This field termination is conventionally carried out by mechanical cleaving and so non-planar, angled ends faces which offer low back-reflection and low insertion loss are only useful if they can easily be created in the field, and therefore, preferably, they should be created by mechanical cleaving.
Murgatroyd (U.S. Pat. No. 6,576,747) reveals (e.g. FIG. 1a and FIG. 1b) a technique for angle cleaving optical fiber or fibers by clamping the fiber, bending it between an anvil and a closely spaced-apart edge and scratching the fiber with a sharp blade. Murgatroyd (U.S. Pat. No. 8,069,691) reveals (e.g. FIG. 3b and FIG. 3c) a tool and method for creating bend-to-angle-cleaves by conveniently arranging the clamping surfaces, compliant clamping surfaces, anvil and blade around a common pivot. Similarly U.S. Pat. No. 7,605,045 creates shear stresses using an angularly-movable (“rotatable”) fiber-deflecting member, preferably a rotatable double anvil, which can be widely separated (preferably by more than 1 cm, more preferably at least 2 cm) from each of the fiber clamping members. In these cases, the shape of the angle cleaved end is shown in accompanying FIG. 2a and FIG. 4b and is characterized by an angled fiber in the region of the core and a section of the cleaved end face which is substantially perpendicular to the optic axis close to the initiation scratch X.
Referring to FIG. 2a, the region, V, of the angle cleaved end far from the scratch X which initiates the cleave is seen to be at a large angle from the perpendicular with a characteristic distance AROLL-OFF. It is desirable that this distance is preferably less than 30 μm for a 125 micron diameter single-mode fiber and always less than 50 μm. If AROLL-OFF is too large, angle cleaved fibers may not splice together because the absence of glass may effect the seating of the cleaved fiber in a splice and so lead to excess insertion loss. Generally, AROLL-OFF is decreased if greater tension is present in the fiber.
An equi-angular plot of the cleaved end face is shown in FIG. 2c. The maximum protrusion of the glass beyond the core of the fiber is seen to be a distance of A1 in FIG. 2a, with a value of approximately 3 μm. When an optical splice is created by butting together two such angle cleaved ends in a V-groove 3 (see FIG. 6d), the separation of the core can be as large as 2×A1, i.e. about 6 μm, as shown in FIG. 6a. When the gap between the optical fiber is filled with index-matching gel, the insertion loss is approximately 1 dB or less and this is effective in producing a field-installable optical splice with low back-reflection and insertion loss of less than 1 dB. With such angle-to-bend angled cleaved ends, it is therefore less important to control the relative orientation of the angle cleaved ends.
In contrast, if a substantially planar angled cleave is used, as is formed from twisting the fiber prior to cleaving, the gap between the cores can be substantially larger and so the insertion loss is unacceptably high. FIG. 4f shows the shape of the angled end arising from twisting if two such angled ends are brought together in an optical splice in the worst rotational orientation, the gap between the cores can be as large as 2×A t, as shown in FIG. 6f, i.e. 2×(fiber diameter)/2*tan(8°) which is approximately 20 μm. It would be necessary to control the orientation of the angled fibers to reduce this gap if a low insertion loss optical splice were to be made.
The internal stress required to create an angled cleave over the core can be created by bending the fiber as described in U.S. Pat. Nos. 6,578,747, 7,605,045 and 8,069,691. However, it should be understood that the fiber is also significantly stressed by the force required for the diamond to cut in to the glass to the depth of 2-3 μm required to produce the cleave-initiation crack. U.S. Pat. Nos. 6,578,747, 7,605,045 and 8,069,691 all use a diamond blade acting perpendicular to the fiber axis to scratch the stressed optical fiber to initiate a cleave. The cutting force is therefore directed perpendicular to the optic axis of the fiber and so the initial part of the cleave starting from the starter crack (see FIG. 2a), will be substantially perpendicular to the optic axis. In portions of the cleave away from the starter crack, the shearing forces arising from the bend in the fiber encourage the cleaved end face to be angled with respect to the optic axis of the fiber.